Such a motor vehicle brake system is usually provided with two two-way valves that prevent locking of the wheels of a motor vehicle by their alternate switching. The two-way valves are usually driven by means of a control unit designed as a so-called microcontroller. Details on the functioning of a motor vehicle brake system may be gathered, for example, from the specialist book by Dubbel, Taschenbuch für den Maschinenbau [Manual of engineering], Springer Verlag, Heidelberg, 20th Ed. 2001, ISBN 3-540-67777-1, section Q/vehicle engineering, which is incorporated by reference. It is normal to provide a pump for pumping brake fluid in such a motor vehicle brake system. Optimized driving of the two two-way valves, and thus an optimized braking operation, can be achieved by driving the two two-way valves as a function of the brake line pressure between a master brake cylinder and a wheel brake cylinder of a wheel brake. This brake line pressure is denoted below as admission pressure.
DE 10 2005 041 556 A1, which is incorporated by reference, discloses a method for determining an admission pressure prevailing between a master brake cylinder and an inlet valve of a wheel brake cylinder of a motor vehicle brake system. According to the method, in this case the admission pressure is determined by taking account of the profile of a run-on voltage of a motor, operated in a pulsed fashion, of the pump. To this end, a plurality of characteristic variables of the voltage profile are measured and used in each case to determine an admission pressure value, there taking place an evaluation of the quality and/or reliability of the measured characteristic variables, a filtering and/or conditioning of the characteristic variables and/or of the admission pressure values determined therefrom in the event of lack of quality and/or reliability of the measured characteristic variables, averaging of the admission pressure values coming from the different characteristic variables, account being taken only of pressure values of similar magnitude, and the admission pressure values being averaged over time in order to damp fluctuations.
In order to determine the admission pressure during operation of the pump, a correspondingly determined characteristic curve is stored in a data memory of a control unit controlling the motor vehicle brake system. The two two-way valves are driven according to the admission pressure calculated with the aid of the characteristic curve.
However, carrying out the method is rendered difficult by the fact that the characteristic curve turns out to be different for each individual pump and each individual brake system as a consequence of manufacturing tolerances. As the duration of operation of the pump increases, there is, in addition, an occurrence of aging and wear phenomena, for example at the rotary bearing of the pump impeller, and these influence the operating characteristics of the brake system.